DataAssist will not work with multiple key columns.
If your key column is set correctly, there should not be a need to have more than one key column.
The Idea behind a Primary Key Column is to use the AUTO INCREMENT function of the Database to assign a unique number to each record that is inserted. Since the value in the primary key column is unique for each record, you do not need to use multiple key columns to identify the record to update, just use the Primary key column.